The present invention generally relates to a polishing pad for a chemical mechanical polishing apparatus and a method for forming the pad and more particularly, relates to a polishing pad for a linear chemical mechanical polishing apparatus for achieving improved polishing uniformity and a method for forming the pad.
In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, on an inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer which are frequently used in memory devices. The planarization process is important since it enables the use of a high resolution lithographic process to fabricate the next level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer, in a conventional rotary CMP, while the wafer is kept off-center on the pad in order to prevent polishing a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing a tapered profile onto the wafer surface. The axis or rotation of the wafer and the axis of rotation of the pad are deliberately not collinear, however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 xc3x85 and about 10,000 xc3x85. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window etching or plug formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
The important component needed in a CMP process is an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the rotation of the platen. The polishing or the removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure the uniform wetting of the polishing pad and the proper delivery and recovery of the slurry. For a high volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates and different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.
More recently, a new chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefor named as a linear chemical mechanical polishing process in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a uniform polishing rate across a wafer surface throughout a planerization process for uniformly removing a film player of the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus and therefore not only reducing the cost of the apparatus but also reduces the floor space required in a clean room environment.
A typical linear CMP apparatus 10 is shown in FIGS. 1A and 1B. The linear CMP apparatus 10 is utilized for polishing a semi-conductor wafer 24, i.e. a silicon wafer for removing a film layer of either an insulating material or a wafer from the wafer surface. For instance, the film layer to be removed may include insulating materials such as silicon oxide, silicon nitrite or spin-on-glass material or a metal layer such as aluminum, copper or tungsten. Various other materials such as metal alloys or semi-conducting materials such as polysilicon may also be removed.
As shown in FIGS. 1A and 1B, the wafer 24 is mounted on a rotating platform, or wafer holder 18 which rotates at a pre-determined speed. The major difference between the linear polisher 10 and a conventional CMP is that a continuous, or endless belt 12 is utilized instead of a rotating polishing pad. The belt 12 moves in a linear manner in respect to the rotational surface of the wafer 24. The linear belt 12 is mounted in a continuous manner over a pair of rollers 14 which are, in turn, driven by a motor means (not shown) at a pre-determined rotational speed. The rotational motion of the rollers 14 is transformed into a linear motion 26 in respect to the surface of the wafer 24. This is shown in FIG. 1B.
In the linear polisher 10, a polishing pad 30 is adhesively joined to the continuous belt 12 on its outer surface that faces the wafer 24. A polishing assembly 40 is thus formed by the continuous belt 12 and the polishing pad 30 glued thereto. As shown in FIG. 1A, a plurality of polishing pad 30 are utilized which are frequently supplied in rectangular-shaped pieces with a pressure sensitive layer coated on the back side.
The wafer platform 18 and the wafer 24 forms an assembly of a wafer carrier 28. The wafer 24 is normally held in position by a mechanical retainer, commonly known as a retaining ring 16, as shown in FIG. 1B. The major function of the retaining ring 16 is to fix the wafer in position in the wafer carrier 28 during the linear polishing process and thus preventing the wafer from moving horizontally as wafer 24 contacts the polishing pad 30. The wafer carrier 28 is normally operated in a rotational mode such that a more uniform polishing on wafer 24 can be achieved. To further improve the uniformity of linear polishing, a support housing 32 is utilized to provide support to support platen 22 during a polishing process. The support platen 22 provides a supporting platform for the underside of the continuous belt 12 to ensure that the polishing pad 30 makes sufficient contact with the surface of wafer 24 in order to achieve more uniform removal in the surface layer. Typically, the wafer carrier 28 is pressed downwardly against the continuous belt 12 and the polishing pad 30 at a predetermined force such that a suitable polishing rate on the surface of wafer 24 can be obtained. A desirable polishing rate on the wafer surface can therefore by obtained by suitably adjusting forces on the support housing 32, the wafer carrier 28, and the linear speed 26 of the polishing pad 30. A slurry dispenser 20 is further utilized to dispense a slurry solution 34.
In the conventional rotary CMA and linear CMA process, while each presenting certain processing advantages, both techniques suffer from a problem of not being able to maintain polishing uniformity. The lack of polishing uniformity by the rotary CMA technique and the linear CMA technique is caused by a steady decrease in the removal rates of the substrate material after prolonged use of the polishing pad. This is shown in FIGS. 2A-2C and 3A-3B.
FIG. 2A shows a typical rotary CMA polishing pad 50 which is mounted on a polishing platen 52, shown in FIG. 2B. The polishing platen 52 rotates with the polishing pad 50 on top during a rotary polishing process. The removal rate varies with the pad life and decreases significantly as the polishing pad 50 has been used extensively, i.e. for about 200 or 300 wafers.
Similar deterioration in the removal rates with pad life is seen in linear CMP process, as shown in FIGS. 3Axcx9c3C. For instance, FIG. 3A illustrates a linear polishing pad 30 similar to that shown in FIG. 1A. The polishing pad is supported by a pair of rollers 14, also shown in FIG. 1A, for continuous rotation by a motor (not shown). It should be noted that FIG. 3B illustrates a simplified drawing of a linear CMP apparatus and as such, only the polishing pad 30 and the rollers 14 are shown. The dependency of the removal rates on the pad life for the linear CMP method is shown in FIG. 3C illustrating a significant drop in the removal rates after the linear polishing pad had been used extensively, i.e. for polishing between 200 and 300 wafers. The significant drop in the removal rates leads to a polishing uniformity problem and furthermore, leads to the premature replacement of the polishing pad. The more frequent replacement of the polishing pad that is necessary in either the rotary CMP apparatus or the linear CMP apparatus results in decreased yield of the polishing process.
It is therefore an object of the present invention to provide an apparatus and a method for linear polishing that does not have the drawbacks or shortcomings of the continuous belt type linear polishing apparatus.
It is another object of the present invention to provide an apparatus for linear polishing that does not utilize an endless loop of a polishing pad.
It is a further object of the present invention to provide an apparatus for linear polishing that is equipped with a vibration generator for causing the polishing pad to vibrate in a transverse direction of the pad.
It is another further object of the present invention to provide a linear polisher for polishing a substrate consists of a length of a polishing pad supported on a pair of roller means.
It is still another object of the present invention to provide a linear polisher for polishing a substrate consists of a length of a polishing pad, a pair of roller means, a motor means, a vibration generator and a substrate holder.
It is yet another object of the present invention to provide a linear polisher for polishing a substrate that is equipped with a vibration generator for vibrating the pad in a transverse direction in a frequency range between 10/sec and 1,000/sec.
It is still another further object of the present invention to provide a method for linear polishing a substrate that includes the step of vibrating a length of a polishing pad in the transverse direction at a frequency of at least 10/sec.
It is yet another further object of the present invention to provide a method for linear polishing a substrate by vibrating the polishing pad in a transverse direction and rotating a substrate in contact with the polishing pad at a rotational speed of at least 50 RPM.
In accordance with the present invention, an apparatus and a method for linear polishing that does not use an endless loop of a polishing pad are provided.
In a preferred embodiment, a linear polisher for polishing a substrate is provided which includes a length of a polishing pad that has a first end and a second end, a first roller means for removably attaching the first end of the polishing pad thereto and a second roller means for removably attaching the second end of the polishing pad thereto, a motor means for rotating the first roller means such that the polishing pad moves from the second roller means to the first roller means in a longitudinal direction, a vibration generator for causing the polishing pad to vibrate in a transverse direction of the pad, and a substrate holder for mounting the substrate thereto and for pressing an exposed surface of the substrate onto a top surface of the polishing pad.
In the linear polisher for polishing a substrate, the polishing pad may be fabricated of a polymeric material. The first roller means may be a take-up roller for taking up consumed polishing pad and the second roller means may be a storage roller for storing unused polishing pad. The motor means may rotate the first roller means intermittently or continuously. The motor means may rotate the first roller means intermittently after each polishing process for a substrate. The vibration generator may generate vibrations in an ultrasonic frequency range.
In the linear polisher for polishing a substrate, the vibration generator generates vibrations in a frequency range between about 10 cycle/sec and about 1,000 cycle/sec. The vibration generator generates vibrations that have an amplitude between about 0.01 inch and about 1 inch. The polishing pad advances intermittently for a distance of at least xc2xc inch after each polishing process for a substrate is completed. The vibration generator may be coupled to the polishing pad through an adaptor for causing the pad to vibrate in a transverse direction. The substrate holder may be equipped with a pressure means for pressing the substrate onto the polishing pad. The substrate holder may be equipped with a rotation means for rotating the substrate at a rotational speed of at least 50 RPM.
The present invention further discloses a method for linear polishing a substrate which can be carried out by the operating steps of providing a length of a polishing pad that has an abrasive top surface and a length larger than a width. By vibrating the polishing pad in the width (or transverse) direction at a frequency of at least 10 cycle/sec, and pressing a surface of the substrate to be polished on the abrasive top surface of the polishing pad.
The method for linear polishing a substrate may further include the step of rotating the substrate to a rotational speed of at least 50 RPM during the pressing step. The method may further include the step of advancing the length of polishing pad during the linear polishing process. The method may further include the step of advancing the length of polishing pad after a polishing process for a substrate by at least xc2xc inch. The method may further include the step of positioning the length of polishing pad on a pair of roller means. The method may further include the step of rotating at least one of the pair of roller means by a motor means. The method may further include the step of vibrating the polishing pad in the width direction at a frequency between about 10 cycle/sec and about 1,000 cycle/sec. The method may further include the step of vibrating the polishing pad to an amplitude of between about 0.01 inch and about 1 inch.